Pilot
Joe Duff: During the migration
we can tell if the birds are getting tired because they do a number
of things:

1. First
of all there’ll be a lot of changing of the flight order. They’re
looking for an easier place to fly as they get a little tired and
start to panic, I guess, a little bit. . .2. And then you’ll notice that a couple of
birds will open their mouths and start to pant. You’ll see
them panting.3. Then they’ll also drop their legs and splay
their toes so that they help cool their body because it’s starting
to get overheated. 4. And then you’ll see them start to drop
down lower. They’re still trying desperately to keep up but
they’re just having a tough time. And what we can do at that
point is — a second aircraft can come in and pick those birds
up. Now because there are fewer birds in the line (you’ve divided
the flock in half), each aircraft can “carry” an appropriate
number of birds so each bird gets a better ride. The closer they
get to the wing, the easier it is for them. They can fly on that "wake" [of
air current]— so if we have two aircraft, that really helps.

Look! In
the video clip below, you'll see and hear the ultralight coming at
you over the trees, with its vocalizer playing the contact call.
You'll see 3 birds. Then one tires and drops away, and rejoins later.
Do you see the three birds switch positions at the end of the clip?